Catastrophic usb communication failure will make the operating system reset the device and put it on another usb minor number. When this happens, cgminer will hotplug it as a new device - i.e. AVA1 and AVA0 will remain as a zombie device. The web GUI and Avalon operating system environment is not smart enough and may not know that it's still mining at a new location and would just kill off the cgminer whereas mining via USB you'll see that in all its glory.
Would this explain this...
Wed Nov 27 13:48:01 2013 cron.info crond[551]: crond: USER root pid 16293 cmd /usr/bin/cgminer-monitor
Wed Nov 27 13:50:01 2013 cron.info crond[551]: crond: USER root pid 16302 cmd /usr/bin/cgminer-monitor
Wed Nov 27 13:50:28 2013 kern.info kernel: [59485.070000] usb 1-1: USB disconnect, device number 10
Wed Nov 27 13:50:28 2013 kern.info kernel: [59485.080000] usb 1-1.1: USB disconnect, device number 11
Wed Nov 27 13:50:29 2013 kern.info kernel: [59485.380000] usb 1-1: new high-speed USB device number 12 using ehci-platform
Wed Nov 27 13:50:29 2013 kern.info kernel: [59485.540000] hub 1-1:1.0: USB hub found
Wed Nov 27 13:50:29 2013 kern.info kernel: [59485.540000] hub 1-1:1.0: 2 ports detected
Wed Nov 27 13:50:29 2013 kern.info kernel: [59485.820000] usb 1-1.1: new full-speed USB device number 13 using ehci-platform
Wed Nov 27 13:52:01 2013 cron.info crond[551]: crond: USER root pid 16329 cmd /usr/bin/cgminer-monitor
Wed Nov 27 13:54:01 2013 cron.info crond[551]: crond: USER root pid 16340 cmd /usr/bin/cgminer-monitor
Every couple of hours I see the usb reset like this on the TPlink.. is this purely from the tplink??, if so I'll just replace it with a PI, or this more to do with the unit's themselves (i'm running a 4 unit , unsure of batch)